Among tube type shock absorbers attached to suspension systems of automobiles or other vehicles, there is a shock absorber having a structure in which a valve mechanism serving as a damping force generating device is housed in a circular cylindrical casing joined perpendicularly to a side surface portion of a cylinder part by welding or the like, as disclosed, for example, in Patent Document 1. In such a shock absorber, conventionally, the circular cylindrical casing is produced from a pipe material, and a joint surface of the casing that is joined to the cylinder part is machined into a curved surface curved along the outer peripheral surface of the cylinder part.